Plasma processing of semiconductor wafers or other types of workpieces, such as plasma displays, solar panels or photolithographic masks, is employed to perform etch processes, deposition processes and plasma immersion ion implantation processes, for example. The plasma source may be an RF source having a capacitively coupled source power applicator such as a ceiling electrode or an inductively coupled source power applicator such as a coil antenna. The rate at which the plasma process is carried out is typically different at different locations on the workpiece surface. For example, a plasma etch process may have a non-uniform etch rate distribution across the workpiece surface. It is critical that the process rate distribution across a workpiece surface be as uniform as possible. This is particularly true in the fabrication of integrated circuits on a semiconductor wafer, in which critical dimensions of thin film structures have been reduced to 32 nanometers or less. At such small feature sizes, a non-uniform etch rate distribution can cause etch stop in some portions of the wafer and over-etching in other portions of the wafer.
Such process rate distribution non-uniformities may have a radial pattern, in which the spatial distribution of etch rate across the workpiece surface is center-high or center-low, for example. Such radial non-uniformity can be ameliorated or eliminated by employing independently controlled RF source power applicators at different radial locations relative to the plasma reactor chamber. For example, in the case of an inductively coupled RF source power applicator overlying the plasma chamber ceiling, the RF power applicator may consist of inner and outer coil antennas overlying radially inner and outer zones of the workpiece. Radial non-uniformity in distribution of plasma process rate is removed or minimized by adjusting the relative amounts of RF power applied to the inner and outer coil antennas.
The process rate distribution may also have an azimuthal pattern of non-uniformity, which may be referred to as “skew”, in which the process rate varies with the rotational angle about the central axis of symmetry of the workpiece and reactor chamber. Because the skew pattern is inherently asymmetrical, it is challenging to provide an RF source power applicator that may be adjusted to remove or minimize a particular skew pattern exhibited by a plasma reactor.